This is a problem of logic, rather than Perl. The number 55 matches all three of your >= criteria, so you will of course get all 3 outputs. If you only want one output for each then consider reversing the order of your tests and using elsif for the other blocks. eg:

if ($number >= 50 ) {
    # ...
} elsif ($number >= 20) {
    # ...
} elsif ($number >= 10) {
    # ...
}
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This way you will get, at most, one line of output for any value of $number.